Echoing other posters, if the itineraries are close enough for you I'd recommend doing a little thinking about what you expect out of a cruise and then learn a little about each line and ship. Go to the bookstore or library and find books that provide general information on cruising. They usually have profiles of "typical cruisers" on each line - though nobody claims to be typical
My gut says you would enjoy either cruise.
All I ask is a tall ship and a star to steer her by; John Masefield
Carnival: Glory 2004, Destiny 2008, Splendor 2009, Freedom 2011, Valor 2012, Dream 2013
Celebrity: Summit 2011
Princess: Ruby 2010, 2014, Caribbean 2013, Coral 2014, Regal 2014
Star Clippers: Royal Clipper 2015